1. Field of the Invention
The present invention relates to an optical writing apparatus and an image forming apparatus including the optical writing apparatus.
2. Discussed of the Related Art
Typically, an image forming apparatus using electro-photography includes an optical writing apparatus for writing an electrostatic latent image on an image carrying member (e.g., a photoconductive member).
The optical writing apparatus includes an optical system having a plurality of optical elements such as light source unit, a polygon mirror, a scanning lens, a reflection mirror, and a casing to support and encase such optical system in a dustproof manner.
Conventionally, such casing is made from a resinous material to provide a light-weight and low cost structure, or from a material having a low line expansion coefficient (e.g., metal) to suppress a deformation of the casing due to a temperature change.
As for an optical writing apparatus having a casing made from a resinous material, such casing expands and contracts in response to a temperature change.
The expansion or contraction of the casing wall leads to displacement of optical elements in the casing from their original positions, resulting in a change of distance (i.e., change of relative position) between the optical elements.
For example, a light beam emitted from a semiconductor laser in a light source unit may not be irradiated at an adequate position on an image carrying member when a relative position between the optical elements is changed.
As for an optical writing apparatus having a casing made from a metal, expansion or contraction of such casing in response to a temperature change is relatively small compared to the casing made from a resinous material, resulting in a reduction of the change of the relative position between the optical elements in the casing. However, the casing made from the metal increases its manufacturing cost and weight of the optical writing apparatus.